4
HARNESSING
ENERGY FLOWS:
TECHNOLOGIES FOR
RENEWABLE POWER
PRODUCTION
Ola Carlson
Linus Hammar
Zack Norwood
Emil Nyholm
Department of Energy and Environment , Chalmers University of Technology*
Systems Perspectives on Renewable Power 2014, ISBN 978-91-980974-0-5
* Division of Energy Technology (Z. Norwood, E. Nyholm), Division of Electrical Power Engineering (O. Carlson),
Division of Environmental Systems Analysis (L. Hammar)
Chapter reviewers: Björn Sandén, Environmental Systems Analysis, Fredrik Hedenus, Physical Resource Theory,
Energy and Environment, Chalmers
INTRODUCTION
In this chapter, the technologies for renewable power production of today and
in the near future will be described and explained. Renewable power production
is electric power production without using a fuel that will end some day in the
future. In this chapter, as in this book, power production based on renewable fuels
(biomass) is excluded1.
Some of the technologies such as wind or solar has reached industry mass
production in recent years, hydro power has been in operation more than 100
years and others like wave or ocean current power have still some development to
do before robust power production units are available.
SOLAR POWER
Solar energy is harnessed today by two main types of technology. Thermal-electric
systems, often called concentrating solar power (CSP), collect the light from the
sun and convert that thermal energy to electricity through a heat engine, whereas
1 For more details on this topic, see Systems Perspectives on Biorefineries. (2014) 3rd edition. Chalmers Univeristy of Technology, Göteborg, Sweden.
32
photovoltaic (PV) systems convert the photons from sunlight directly into electricity
in a semiconductor device. Although the photovoltaic process is more direct, the
overall efficiency (percent of sunlight incident that is converted to electricity) of
commercial solar thermal-electric and photovoltaic systems fall in similar ranges
(10-30%), with the high end of this range reached for both high concentration PV
systems and some CSP systems.
All solar power technologies use electromagnetic radiation from the sun to generate electricity, but if a system optically concentrates the light it collects primarily
the direct portion of the radiation, whereas non-concentrating systems (e.g. flat
plate PV) can collect both the direct and diffuse components of sunlight (see
Chapter 3).
Additionally, since only direct light can be optically concentrated, concentration
requires the ability to track the sun so that the collector is always pointing directly
at the sun as it moves across the sky, further complicating the system. However,
since solar thermal-electric efficiency benefits greatly from generating higher
temperatures to drive the heat engines that convert the thermal energy to electricity, concentrating systems are the standard in this arena.
In the case of photovoltaics, there is also a potential, due to the properties of the
PV cell material, to increase efficiency and substantially decrease the needed
amount of the sometimes expensive photovoltaic material by using concentration,
typically with exotic multi-junction high-efficiency solar cells. The economics of
concentrating PV (CPV) are not as favourable as in the CSP case, because CPV
increases the need for cooling, in addition to the tracking and more complex optics
required, and there is typically not as strong an increase in efficiency with concentration as in thermal systems.
Metal (front contact)
Antireflective (AR) coating
Sunlight
n+ layer
p- type layer
p+ type layer
Aluminium (back contact)
Figure 4.1 A crystalline silicon solar cell.
At the core of photovoltaic technology is the solar cell, or the material that converts the sunlight to electricity. A solar cell is formed at the junction between two
semiconductor materials (of which there exists many varieties). Multiple such junctions can be arranged in series (or parallel) that have different abilities to absorb
different wavelengths of light (corresponding to different electron band gaps). All
33
of these variations, in the end, affect how much of the sunlight can be converted
to electricity, with the goal to develop low-cost materials reaching the theoretical
limit of efficiency. For a single junction cell, as shown in Figure 4.1, this efficiency
limit is approximately 30%, but increases to 42% for two-junctions, and 48% for
three-junctions, with a theoretical limit of 68% achievable with infinite junctions.
Under high concentration the corresponding limits are 40% for a single-junction
cell, 55% for two-junctions, 63% for three-junctions, and an 86% theoretical limit
with infinite junctions.2
The PV industry is dominated by silicon cells. Silicon technologies are broadly
divided into crystalline cells (single or polycrystalline), which make up over 80%
of the world market,3 and non-crystalline cells (amorphous). Amorphous cells are
generally thin-films, meaning a thin layer (about one micrometer) of the semiconductor material is deposited on a base layer. This process reduces cost by reducing the amount of material used in the process, but also decreases the efficiency
of the cell compared to crystalline silicon cells. Cadmium telluride (CdTe) and
copper indium gallium selenide (CIGS) cells are other examples of commercial
thin film technology. At the top end of the spectrum, in terms of efficiency, are
multi-junction cells, the most advanced of which are generally made up of layers
of compounds of group III and V elements of the periodic table. A range of other
types of cells are under development including dye-sensitised, organic, and
quantum dot solar cells.
When talking about systems that convert sunlight to thermal energy and then to
electricity, we often use the term “concentrating solar power” (CSP) although, as
mentioned above, these systems could also focus the sunlight on PV cells instead
of a thermal fluid. The scale of CSP systems is usually very large (i.e. power plant),
but smaller systems can also be designed, for example, in remote villages for rural
electrification. Solar thermal-electric systems offer the advantages of being suitable for operation on other fuels when the sun is not shining, and can store energy
as thermal energy to later be converted to electricity. This method of storing
energy thermally is generally less expensive than storing the generated electricity
at a later stage (see also Chapter 5 and 12).
The general principle behind solar thermal-electric systems is that a working fluid
(usually a molten salt, mineral oil, or water) is heated to high temperatures at the
focus of a concentrating solar collector, and the energy from that hot fluid is then
used to run a heat engine. The heat engine is usually based on either a Rankine
cycle (the same cycle used in most fossil-fuel power plants) or a Stirling cycle.
To get the high temperatures needed to operate heat engines efficiently, solar
thermal-electric systems usually use concentrating solar collectors which can
produce fluid temperatures from a couple hundred to over a thousand degrees
Celsius. These collector systems can generally be categorised as one of four
types: Parabolic trough, linear Fresnel, dish engines, or central receivers, as
shown in Figure 4.2.
2 De Vos, A. (1980) Detailed balance limit of the efficiency of tandem solar cells. Journal of Physics D: Applied Physics
13(5):839.
3 Masson, G, M et al. (2013) Global market outlook for photovoltaics 2013-2017. Brussels, Belgium: European Photovoltaic
Industry Association (EPIA).
34
Parabolic trough
Reflector
Dishengine
Solar receiver
Linear fresnel
Central receiver
Sun light
Figure 4.2 Types of CSP (Concentrating Solar Power).
An area of expanding research in the field of solar power is so called hybrid
PV-thermal (PVT) systems. These systems combine a thermodynamic heat engine
cycle, like in CSP, with a photovoltaic material to boost the overall conversion
efficiency of sunlight to electricity. For example, one such system would use an
optically selective fluid (e.g. with suspended nanoparticles) running over a photovoltaic material at the focus of a concentrating solar collector. The fluid would
mainly absorb those wavelengths of light that were not useful to the PV, thereby
allowing the useful wavelengths to hit the PV, while the other wavelengths heat the
thermal fluid to high enough temperatures to run an additional heat engine cycle to
produce electricity. The overall solar-electric efficiency from such a system could
be higher than either a CSP or PV system alone.
The output of all solar power systems varies directly with the amount of sunlight,
so is highest during the summer (on the northern hemisphere), tapers off in the
winter, and varies depending on seasonal weather patterns. Regions nearer to the
equator see less variation and more total production.
Solar electricity production in Germany 2013
GW
25
15
5
Jan
Feb
Mar
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Oct
Nov
Dec
Wind electricity production in Germany 2013
25
GW
Apr
15
5
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Figure 4.3 Solar electricity production in Germany, 2013 (top) compared to wind electricity production in the same
year (bottom). Source: Fraunhofer ISE (2013).
Figure 4.3 shows the total production of solar electricity in Germany in 2013
compared to the corresponding production from wind turbines in that year. Note
that the seasonal variation of these technologies makes them good complements
35
for each other, as wind is often stronger in the winter months and solar in the
summer months.
As of 2013 PV make up the lion’s share of existing solar-electric capacity, with
global PV capacity estimated at 140 GW, and thermal-electric at less than 3 GW.
The solar electricity production the same year can be estimated at 130-150 TWh.4
In the last decade, the installed PV capacity has grown, on average, by more than
50% annually.
WIND POWER
Wind power turbines create electrical energy from the kinetic energy in the wind.
A wind power plant operates according to following principle: 1) The motion of the
wind puts the turbine in motion; 2) a torque is created on the axis connected to
the generator; and 3) the generator transforms mechanical energy into electrical
energy.
Rotor blades
Gearbox
Anemometer
Genrator
Yaw motor
Power grid
Control system
Concrete foundation
Transformer
Figure 4.4 Components of a wind power plant.
A wind power plant consists of a number of components, shown in Figure 4.4. The
main components are foundation, tower, wind turbine and nacelle. The foundation
gives stability to the plant construction. The tower is often created by steel and
has the shape of a cone. Some fabricates has concrete as an alternative material
for the tower. The nacelle contains gearbox, generator and electrical equipment,
and it turns towards the wind direction. There are several ways to design a wind
power plant. Size of tower, rotor blades and the electrical system varies between
different models.5
4 The rapid growth in many different countries with varying insolation makes the estimate uncertain. The estimate is based on
BP (2013) BP statistical review of world energy 2013. London, UK: BP plc.
5 Wizelius, T. (2002) Vindkraft i teori och praktik. Lund, Sweden: Studentlitteratur.
36
The kinetic power of the wind is proportional to the density of the air mass and the
third power of the wind speed (see Box 4.1). The implication of this is that when
the wind speed doubles, the power increases by a factor of eight. Hence, the wind
energy resource varies a lot between windy and less windy locations (see Chapter
5).
Box 4.1 Power and tip speed ratio.
The power from a turbine, P(W), is calculated from:
P = cp Pw =
1
c ρ A vw3
2 p
where Pw = kinetic power of the fluid (wind or water current), ρ = density
of the fluid (kg/m3), A = wrapped rotor area (m2), vw = undisturbed wind
speed (m/s), cp = efficiency coefficient = 16/27 η, where 16/27 is the
theoretical maximum efficiency (the Betz limit) and η = the efficiency of
the turbine, cp is usually 0.4 – 0.5 for wind turbines.
The tip speed ratio λ, measures of how fast the blades rotate compared
to the speed of the fluid (wind or water)
λ=
vb
ωr
=
vw
vw
where vb = tip speed of the blades (m/s), r = radius of the rotor (m), and
ω = angular velocity (rad/s).
When a wind turbine is designed, the tip speed ratio is an important parameter. It
is a measure of how fast the blades rotate compared to the wind speed (Box 4.1).
If the wind turbine rotates too slowly, most of the wind will pass the rotor without
hitting the blades. On the other hand, if the rotor speed is too fast, the wind will
have a hard time passing the rotor, since the rotating blades will act like a wall
against the wind. Because of this, the wind turbine is designed according to an
optimal value of the tip speed ratio, to extract a maximum amount of energy. The
theoretical maximum efficiency of a turbine, the Betz limit, is about 59% (or exactly
16/27).
For technical reasons, each power plant has maximum power output, i.e. it cannot
make use of the all the energy of wind speeds above a rated speed. Figure 4.5
shows the theoretical wind power curve and a curve measured from a wind power
plant owned by Chalmers University of Technology and situated on the island of
Hönö, outside Göteborg, Sweden. The measured values initially follow the theoretical curve, but as the rated wind speed is approached, the measured power
output stabilises at a constant value. At this point, the turbine, not the available
wind energy, limits the power output.
37
Power curve
Mechanical power (kW)
30
20
Theoretical
10
Measured
0
0
15
10
5
Wind speed (m/s)
Figure 4.5 Theoretical and measured power curve from Chalmers wind power plant on the island of Hönö, Sweden.
There are mainly two types of wind power plants, vertical axis wind turbines,
VAWT, and horizontal axis wind turbines, HAWT. In the VAWT, the axis between
the generator and turbine is vertical to the ground, and in the HAWT it is horizontal
to the ground. The HAWT dominates the wind power market today. Figure 4.6
shows one traditional (A) and two modern (B and D) HAWTs, and one VAWT (C),
and their performance profiles.
0.5926
A turbine free of losses
0.5
B
D
0.4
A
C
Cp 0.3
B
A
0.2
0.1
0
C
0
2
4
6
8
10
12
14
16
D
18
y
Figure 4.6 Different types of wind turbines.
At the end of 2013, the accumulated wind power installation worldwide was 310
GW and the electricity production in 2012 was about 500 TWh.6 The installed
6 GWEC (2014) Global installed wind power capacity in 2013 - Regional Distribution. Global Wind Energy Council;
BP (2013) BP statistical review of world energy 2013. London, UK: BP plc.
38
capacity has on average grown by about 25% annually over the last three decades. In the last decade, the European offshore market has grown rapidly, from a
few MWs to an installed capacity of more than 6 GW in 2013.7
HYDRO POWER
Hydro power, originating from the early civilizations millennia ago, exploits the
potential energy of water precipitated on land at altitudes higher than sea level by
forcing water from rivers or reservoirs through turbines. The available amount of
energy is dependent on water flow (m3/s) and the head (m) (water level difference).
Historically, the mechanical energy was used directly for milling and other machinery. Today, generators convert the energy into electricity.
Several different turbine designs are used. Two common turbines are the Francis
turbine and the Kaplan turbine where the former is the most common for highhead systems and the latter is typically used in low-head systems. Hydropower
schemes can be of different types: Run-of-river schemes have turbines installed
directly in a river, or have pipes leading water from a river through an adjacent
turbine installation thus harvesting energy from the natural flow without options for
energy storage. Storage schemes have a dam which impounds water and turbines
installed in the dam wall, hereby partly controlling the temporal availability of the
energy resource. Pumped-storage schemes have several interconnected reservoirs so that water can be pumped from lower to higher reservoirs when electricity
demand is low, hereby allowing for a high control of the temporal availability of the
energy resource.
The different schemes vary in their ability of storing energy and thereby optimising the selling price. Storage- and pumped storage schemes can be useful base
supply in electric grids where power is saved for peak load periods (see Chapter
11), while run-off-river schemes, which are smaller, have the advantages of simple
installation, lower environmental impacts and higher geographical availability.
Although large-scale hydropower contributes with the vast majority of generated
power, small-scale hydropower is regarded increasingly important for remote area
power supply in many countries.8
Hydropower production has grown almost linearly over four decades, from
1000 TWh per year in the 1960s to 3700 TWh in 2012. Since 2000, its share of
world electricity production has remained at about 16%.9
TIDAL POWER
Tidal power comprises both tidal barrages and tidal current turbines. Barrages
capture the tidal wave inside large enclosures that are open when the tide rises
and closes at high tide. With tidal withdrawal, at ebb, a head is created between
the water trapped inside the barrage and the natural sea level outside the barrage.
Electricity is generated when the water levels are allowed to even out through
low-head turbines.
7 EWEA (2014) The European offshore wind industry - key trends and statistics 2013. Brussels, Belgium: European Wind
Energy Association. See also Chapter 15.
8 Kumar, A. et al. (2011) Hydropower. In IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation
(eds O. Edenhofer, et al.):437-496. Cambridge, UK and New York, USA: Cambridge University Press
9 BP (2013).
39
Tidal barrages can operate through one-way mode, where electricity is only
produced during ebb, or two-way mode, where electricity is generated during
both flood and ebb.10 Tidal barrages are normally constructed across natural bays
or river inlets in order to minimise the length and cost of the barrage. Modern
tidal barrages may also come to be constructed on offshore banks (called tidal
lagoons). Tidal barrages are based on conventional hydropower technology and
can be used both for supplying local needs through small tidal ponds and for large
scale power production.11
Tidal barrage technology is proven since more than 50 years but only a handful of
large barrages have been installed. The most well-known being La Rance power
station (240 MW) in France but the technology is used also in e.g. South Korea,
China, Canada and Russia. A very large tidal barrage is now under appraisal in the
Severn Estuary, UK.12 A mean tidal range of 5 m is often thought to be required for
tidal barrages to be economically feasible, but with modern low-head turbines also
lower tides may be of interest.13 Tidal barrages imply modification of the natural
tidal regime, inevitably affecting the local environment. The environmental impact
of tidal barrages has long been considered a major constraint to expansion of the
technology (Chapter 8).
Tidal current turbines utilises energy from the fast-flowing currents that develop
where the tidal wave passes through narrow straits or bends around peninsulas.
A large number of technical solutions for tidal current power are currently under
development. Electricity is generated through submerged turbines typically driven
by large horizontal-axis rotors, much looking like underwater wind power plants,
or small vertical-axis rotors mounted in grid-like constructions. Some designs are
also based on oscillating hydrofoils.14 On smaller turbines the water flow over
rotors can be enhanced by ducting shrouds.
As with wind power and wave power, tidal current power is based on relatively
small units (<2 MW) and meant for deployment in arrays. Most of these modern
devices are open-flow tidal current turbines targeting tidal currents with water
speeds around 2-4 m/s. The energetic currents also imply harsh conditions and
rise high demand on marine engineering. Nevertheless, several full-scale devices
have recently been installed and the technology is often believed to stand a good
chance of becoming locally important in the near future.15
The tidal resource is dependent on the tidal range (m) which varies among locations due to landmass positioning and local bathymetry. Fast-flowing currents are
rare and tidal current power is therefore particularly site specific (see Chapter
10 Charlier, R.H. (2003) Sustainable co-generation from the tides: A review. Renewable and Sustainable Energy Reviews,
7(3):187-213.
11 Charlier, R. H. and Justus, J. R. (1993) Ocean energies. Amsterdam, The Netherlands: Elsevier Science Publishers.
12 Xia, J. et al. (2010) Impact of different tidal renewable energy projects on the hydrodynamic processes in the Severn Estuary,
UK. Ocean Modelling, 32(1-2):86-104.
13 Liu, L. et al. (2011) The development and application practice of neglected tidal energy in China. Renewable and Sustainable
Energy Reviews, 15(2):1089-1097.
14 Khan, J. and Bhuyan, G. (2009). Ocean Energy: Global Technology Development Status. (T0104) IEA-OES. [Online],
15 Bahaj, A.S. (2011) Generating electricity from the oceans. Renewable and Sustainable Energy Reviews, 15(7):3399-3416;
Esteban, M. and Leary, D. (2012) Current developments and future prospects of offshore wind and ocean energy. Applied Energy,
90(1). pp. 128-136.
40
3). The tidal period is 24 h 50 min and the tide rises and falls one (diurnal tides)
or two (semi-diurnal tides) times per day. In addition, the magnitude of tides
increases two times per month (spring tides) as the gravitational force of the sun
complements the force of the moon. The tidal power output is therefore variable
over both hours and weeks, unlike the phase of human demand which typically has
a diurnal period of 24 h, sometimes in addition to an annual period. By tidal barrages this can partly be solved by dividing the barrage into different basins so that
the outflow is regulated and at least the diurnal variation is reduced. The output of
tidal current turbines cannot be regulated and power remains variable but highly
predictable.
OCEAN CURRENT POWER
Ocean current power, or ocean current energy conversion, basically works under
similar principles as tidal current power, with turbines capturing the energy of the
flow. However, due to the low energy density of ocean currents the power devices
either have to be very large or particularly ingenious in the design (the power in
stream is proportional to the third power of the water speed, see Box 4.1 and Figure 3.2). As an example of the former, it has previously been suggested to extract
power from the Florida Current by installing arrays of turbines with rotor diameters
exceeding 100 m.16 A modern example of the latter is the Deep Green prototype,
currently being developed by the Swedish firm Minesto17. The Deep Green
consists of an underwater kite equipped by a smaller rotor and turbine. As the kite
is swept in circles by the current the rotor experience an increased water flow,
enhancing the energy density by a factor of ten. Ocean current power devices are
still in early development and it is not known whether marine fauna will be able to
avoid collision with its moving components. Thus, both costs and environmental
feasibility of this power source remain very uncertain (Chapter 8).
WAVE POWER
Wave power uses wind-driven surface waves to generate electricity. The many
currently developing technical solutions can be classified as oscillating water
column systems where waves pressurise air chambers and spin turbines, absorber
systems where a buoy attached to the seabed is dragged up and down by the
waves in order to spin turbines or drag pistons through linear generators, overtopping systems where waves force water into an elevated reservoir which is emptied
through low-head turbines, inverted pendulum systems where waves force an
oscillator to move back and forth and pressurise fluids to drive generators, or
elongated attenuators (interconnected elongated floaters) where the wave motions
pressurise hydraulics connected to internal generators.18
Wave power devices can be shore-based, installed in shallow water, or anchored
in deeper water. Floating wave power units are small (<1 MW) but intended for
array deployment. As a general indicator wave power devices harvest about a fifth
of the incoming wave energy and some devices are more generalist than other in
16 Charlier, R. H. and Justus, J. R. (1993) Ocean energies. Amsterdam, The Netherlands: Elsevier Science Publishers.
17The Deep Green turbine is developed by Minesto. See Minesto (2014).
18 Thomas, G. (2008). The Theory Behind the Conversion of Ocean Wave Energy: a Review. In Ocean Wave Energy - Current
Status and Future Perspectives (ed. J. Cruz):41-91. Heidelberg, Germany: Springer (Green Energy and Technology); Khan, J. and
Bhuyan, G. (2009). Ocean Energy: Global Technology Development Status. (T0104) IEA-OES. [Online]
41
the capturing of variable frequencies of waves. The power output is variable and
undergoes both seasonal and daily changes, but is typically less variable and more
predictable than wind power.19 Long-distance waves (swell), which characterise
tropical oceans, even out much of the short-term variation and thus, wave power
around tropical islands can be particularly suitable. A common problem for wave
power systems, and particularly for offshore devices, is to be sensitive enough
to efficiently utilise common waves and at the same time be robust enough to
withstand the rare but powerful extreme waves. Environmental impacts of wave
power can be expected to be limited unless a high proportion of the incoming
wave energy is absorbed by the power plants (Chapter 8).
OCEAN THERMAL ENERGY CONVERSION
Ocean thermal energy conversion (OTEC) utilises the temperature difference
between warm surface water and cold deep sea water using heat engine technology.20 Surface water and deep sea water are collected from the ocean via large
diameter pipes and then released back to the ocean or partly utilised as freshwater. Electricity can be generated using open-cycle, closed-cycle or hybrid designs.
In an open-cycle OTEC warm surface water is vaporised in low pressure chambers and used to drive turbines before it is re-condensed by the cold deep sea
water. In a closed-cycle OTEC warm surface water heats up a working fluid that
vaporises and drives turbines before it is re-condensed by the cold deep sea water
and recycled in the process. In the hybrid design warm surface water is vaporised
like in the open-cycle design and is then used to vaporise a working fluid which
in turn drives turbines. In the open-cycle and hybrid cycle designs, freshwater is
produced as a by-product. This can be an important additional value where water
for consumption or irrigation is scarce. For instance, a small OTEC plant has been
installed in India with the sole purpose of producing freshwater21 and a full scale
100 MW OTEC could produce freshwater enough for a larger city.
OTEC power plants can be installed on land or offshore as ship-like constructions.
Due to the requirement of massive amounts of seawater, only large scale plants
can become economically viable. By adding solar heaters to warm up the surface
water the efficiency of the process can be improved.22 The OTEC resource is
determined by water temperature differences (Figure 3.3 and 3.4) and its availability, which in the case of land-based OTEC is determined by the distance between
shore and deep sea water (1000 m depth is often assumed to be enough). As the
heating and currents are relatively constant, OTEC power production is predictable although it may vary slightly over seasons.23 The OTEC principles where
explored with several pilot plants already in the mid-20th century, so the technical
19 Charlier, R. H. and Justus, J. R. (1993) Ocean energies. Amsterdam, The Netherlands: Elsevier Science Publishers.
20 Krock, H. (2010). Ocean Thermal Energy Conversion. In 2010 Survey of Energy Resources (ed. P. Gadonneix):588-602.
London, UK: World Energy Council; Magesh, R. (2010). OTEC Technology - A World of Clean Energy and Water. Proceedings of
the World Congress on Engineering, 2. WCE 2010, London, U.K., Jun. 30 - Jul. 2.
21 Bhuyan, G.S. (2008). Harnessing the Power of the Oceans. IEA OPEN Energy Technology Bulletin, 52:1-6.
22 Straatman, P.J.T. and van Sark, W.G.J.H.M. (2008). A new hybrid ocean thermal energy conversion-Offshore solar pond
(OTEC-OSP) design: A cost optimization approach. Solar Energy, 82:520-527.
23 Vega, L.A. (2011) Hawaii National Marine Renewable Energy Center (HINMREC). OCEANS 2011 :1-4, Waikoloa, HI, USA,
Sep. 19-22
42
principles are not new. Its implementation has been haltered due to high capital
costs of construction, however, a small OTEC plant is now under construction off
the coast in the French island of Martinique. The environmental impact could be
considerable (Chapter 3 and 8).
GEOTHERMAL POWER
Geothermal power utilises the heat produced within the earth in order to generate
power, in contrast to most other forms of renewable power for which the primary
energy source is the sun (see Chapter 3 and Table 3.1).
There are several geothermal resources that could be utilised as heat sources.
However, today hydrothermal sources in the form of vapour and hot water at
depths up to a few kilometres are the only commercially used sources. In these,
water is used as an energy carrier, moving energy from the Earth’s interior to the
surface. Such resources are primarily created by rain water which percolates
down to areas of permeable heated rock, there the water is heated acting as an
energy reservoir. On top of the reservoir there needs to be a cap rock, i.e. an
impermeable rock that prevent vapour from rising to the surface. In order to utilise
the energy, wells are drilled to the reservoirs and through these the hot water or
vapour rises to the surface.
At the surface, the heat energy is used to generate electricity and the water, now
at a lower temperature, is then injected back into the reservoir. Depending on
water temperature of the field three different methods are used to generate electricity: flash-steam, dry-steam and binary cycles. All of these have capacity factors
of up to 0.9 (new plants) with an average of 0.75 and are thus able to act as base
load plants.24 The size of a power plant can vary between 0.1 MW and 120 MW.25
The most common type is the flash steam. For such cycles the fluid from the well
is used as working fluid in a steam cycle. In the process pressurised water rises
from the well and as the pressure drops the water eventually flashes into steam.
The steam and hot water are then separated and the steam is used to drive a
steam turbine. Due to the prevalence of corrosive gases in the steam the relatively
low temperature, resulting in water droplets being formed, the turbine needs to
be both corrosion and erosion resistant, and therefore more costly than ordinary
turbines.
Dry-steam plants are used in areas which can produce dry superheated steam as
a source. This removes the need for a flash process in order to separate liquid and
vapour, resulting in a less complex and thereby cheaper plant. Steam is instead fed
to a particulate remover and then directly to the turbine
24 Goldstien, B. et al. (2011) Geothermal Energy. In IPCC Special Report on Renewable Energy Sources and Climate Change
Mitigation, Cambridge, UK and New York, USA: Cambridge University Press; IEA-GIA (2012) Trends in geothermal applications,
Survey Report on Geothermal Utilization and Development in IEA-GIA Member Countries in 2010. Taupo, New Zealand: IEA
Geothermal Implementing Agreement
25 DiPippo, R. (2012) Geothermal Power Plants - Principles, Applications, Case Studies and Environmental Impact 3rd Edition.
OXFORD, UK: Butterworth-Heinemann.
43
Generator
Condenser
Separator
Wellhead
Steam
Water
Water
Turbine
Ground surface
Wellhead
Subsurface
Injection
Figure 4.8. Principal schematic of a flash steam plant
For temperatures below 150°C it becomes economically difficult to use a flash
process to convert the heat to electricity. Instead a binary cycle, typically an
organic Rankine cycle, is used in which a separate working fluid is heat exchanged
with the fluid from the well and cycled in the power plant. In such a plant a pump
is used to bring the water to the surface and to keep it pressurised enough to
remain a liquid. The water is then heat exchanged with the working fluid with a low
boiling point. The vapour is then fed to a turbine, condensed and recirculated to
the heat exchanger. The water from the well is injected back into the reservoir after
the heat exchanger. There are also more complex forms of binary plants as well as
combined binary and flash steam plants.
The installed capacity of geothermal power plants has increased linearly since the
1970s and reached 12 GW in 2013 with an expected annual production of 70-80
TWh.26
CONCLUDING REMARKS
The electric power production of the world of today is dominated by coal and
gas. As described in Chapter 2, there is an urgent need to transition to renewable power. Renewable power plants are different and come in many forms, rely
on a range of resources and use a variety of conversion technologies. Some rely
on knowledge fields already mainstream in the energy sector, such as the thermodynamic cycles in solar thermal electric and geothermal power plants, others
draw on similar physical principles while applied in different environments, such
as turbines rotating in air or water, while yet others, such as solar cells, bring in
26 GEA (2013). 2013 Geothermal power: International market overview, Washington DC, USA: Geothermal Energy Association.
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the domains of semiconductor electronics and nanotechnology. There are vast
opportunities to capture the energy flows that every second pass by, but it will
require combination of knowledge fields, engineering ingenuity, entrepreneurial
experimentation and continuous investment.
The maturity of the technologies differs widely. While some, like many ocean
energy technologies remain to be extensively tested, hydro power has grown
steadily over more than a century. In the last two decades, wind and solar PV has
grown exponentially, initially from low levels, but now reaching several percent
of the electricity supply in many countries. However, as shown in Chapter 3, all
renewable power technologies are far from their ultimate potentials, and as shown
in the rest of this book, there are many obstacles yet to overcome, related to
compatibility with existing technical systems, environmental concerns, economic
competitiveness and political power.
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